Originally Posted by
A Chozo
Here's the thing, you don't need faith in the Light to use Holy spells - It could be faith in a deity, in a concept, in someone, etc. It triggers by faith and/or willpower (as Benedictus did). If the Tauren interprets the Light as 'the Light of the Earthmother", they'll think that it all comes from her, and it fits their religion.
The most recent example of a race having faith in something other than the Light, and still using Holy magic, are the Zandalari. They had faith in Rezan, and after he died, they still upheld his virtuous ideals, allowing their prelates to keep using Holy magic. For a while they even lost access to it because they lost Faith once Rezan died.
An ancient example of Holy-magic use without any consideration to the Light are the Fel Orcs. Some are capable of simple healing spells that use Holy magic. How? Probably basic willpower, because in the end those are still spells. There's also the Night Elves, who use the Light of Elune - could mean that Elune infuses their kind with Holy magic, and it could also be their faith providing the Holy power. The Light has been behaving this way since Classic.
Also, the Sunwalkers aren't "suddenly" acting through faith. There are extensive discussions between Aponi and Tahu leading to the creation of the Tauren's Priest and Paladin orders.